Underfill film having thermally conductive sheet

ABSTRACT

An underfill film for an electronic device includes a thermally conductive sheet. The electronic device may include a printed circuit board, an electrical component, an underfill, and the thermally conductive sheet. The underfill is situated between the circuit board and the component. The thermally conductive sheet is situated within the underfill, and together with the underfill, constitutes the underfill film. The device may include solder bumps affixing the component to the circuit board, the underfill film having holes within which the solder bumps are aligned. There may be solder bumps on the underside of the circuit board promoting heat dissipation. There may be heat sinks on the circuit board to which the thermally conductive sheet is affixed promoting heat dissipation. The thermally conductive sheet may be affixed to a chassis promoting heat dissipation. The thermally conductive sheet thus promotes heat dissipation from the component to at least the circuit board.

FIELD OF THE INVENTION

The present invention relates generally to underfill films, which aredisposed between electrical components mounted printed circuit boards ofelectronic devices via solder to relax stresses caused by coefficient ofthermal expansion (CTE) differences between the solder and theelectronic components and between the solder and the printed circuitboards. The present invention relates more particularly to suchunderfill films that include thermally conductive sheets to promote heatdissipation from the electrical components, where otherwise theunderfill films have relatively poor thermal conductivity.

BACKGROUND OF THE INVENTION

Electronic devices commonly include printed circuit boards on whichelectrical components, such as integrated circuit (IC's) and other typesof electrical components, are mounted and connected in particular waysto provide desired functionalities. A common approach to mount anelectrical component to a printed circuit board is to use solder. Moreparticularly, a number of solder bumps may be applied to the printedcircuit board and heated, such that an electrical component can then bedisposed to the solder bumps to affix the component to the circuitboard.

Unfortunately, this approach to mounting electrical components toprinted circuit boards can be problematic. Stresses can result from thedifferences in the coefficient of thermal expansion (CTE) of anelectrical component and the CTE of solder, as well as from differencesin the CTE of a printed circuit board and the CTE of the solder. Duringuse of such an electrical device, for instance, if these stresses becometoo high, the solder may crack, causing the electrical component to nolonger be properly affixed to the printed circuit board.

A solution to alleviate this problem is to include an underfill filmbetween the electrical component and the printed circuit board of anelectronic device. The underfill film itself relaxes the stressesresulting from CTE differences of the solder and the electricalcomponent and the printed circuit board. Such stresses are thus absorbedby the film, instead of by the solder, reducing the likelihood that theelectrical component may break way from the printed circuit board.

Another issue within electronic device design is the dissipation ofheat. Modern IC's, for instance, can generate significant amounts ofheat, which if not properly dissipated can cause failure of theirelectronic devices. Furthermore, electronic devices have becomeincreasingly smaller, leading to printed circuit boards that are closelypacked with electrical components. This means that using heat sinks forheat dissipation, as is conventional, can become problematic, becausethey may not be able to be located near the electrical components thatrequire heat dissipation.

One solution is to add solder bumps, or balls, to the underside of aprinted circuit board, which serve to dissipate heat through the printedcircuit board. This approach is not overly effective, however, becausethe printed circuit board itself is usually not a good thermalconductive, such that the printed circuit board itself becomes a thermalinsulator. Therefore, another approach, either alone or in combinationwith solder bumps on the underside of a printed circuit board, is to usethe printed circuit board itself as a type of heat sink to dissipateheat.

For example, printed circuit boards may be manufactured using a resinthat has a high thermal conductivity. Printed circuit boards using suchresins are available, for instance, from Thermagon, Inc., of Cleveland,Ohio. As another example, printed circuit boards may incorporategraphite sheets to improve their thermal conductivity. Such printedcircuit boards are available, for instance, from U-AI Electronics Corp.,of Aichi, Japan. The graphite sheet may be caused to contact the metalor other chassis of the electronic device in question, to furtherimprove heat dissipation, as described in the published Japanese patentapplication no. JP 1999-233904A, published on Aug. 27, 1999, andentitled “Heat release structure print substrate.”

However, in order for a printed circuit board to effectively dissipateheat, there must be a thermally conductive path between the electricalcomponents and the printed circuit board in the first place.Unfortunately, the inclusion of underfill films between the componentsand a printed circuit board effectively results in the components beingthermally insulated from the printed circuit board. That is, mostunderfill films are made of a resin or another material that has lowthermal conductivity. Therefore, heat does not efficiently travel fromthe electrical components to the printed circuit board.

A limited solution is to mix materials with high thermal conductivityinto the underfill film material itself to improve thermal conductivity.For example, underfill available from AI Technology, Inc., of PrincetonJunction, N.J., includes aluminum particles mixed into resin to improvethe thermal conductivity of the resulting underfill film. However, thissolution only transfers heat to the printed circuit board itself. Even aprinted circuit board with a high thermal conductivity still has athermal conductivity lower than most heat sinks, for instance, andtherefore additional heat dissipation may be required. For instance, theprinted circuit boards described above using thermally conductive resinsstill have relatively low thermal conductivity as compared to heatsinks.

For these and other reasons, therefore, there is a need for the presentinvention.

SUMMARY OF THE INVENTION

The present invention relates to an underfill film for an electronicdevice having a thermally conductive sheet. An electronic device of anembodiment of the invention, for instance, includes a printed circuitboard, an electrical component, an underfill, and a thermally conductivesheet. The underfill is situated between the printed circuit board andthe electrical component. The thermally conductive sheet is itselfsituated within the underfill. The underfill and the thermallyconductive sheet make up the underfill film in one embodiment of theinvention.

The electronic device may include solder bumps that affix the electricalcomponent to the printed circuit board, where the underfill and thethermally conductive sheet have holes within which the solder bumps arealigned. There may further be solder bumps on the underside of theprinted circuit board to promote heat dissipation. There may be heatsinks situated on the circuit board to which the thermally conductivesheet is affixed to promote heat dissipation as well. The thermallyconductive sheet may further be affixed to a chassis for the electronicdevice to promote heat dissipation.

The thermally conductive sheet thus promotes heat dissipation from theelectrical component to at least the printed circuit board. Theunderfill itself has relatively poor thermal conductivity, and may be aresin, for instance. By comparison, the thermally conductive sheet hashigh thermal conductivity, and may be a metal sheet, for instance. Thethermally conductive sheet may be a graphite sheet or made from resin,such as silicone, as well. The coefficient of thermal expansion (CTE) ofthe thermally conductive sheet may be substantially equal to the CTE ofthe underfill itself, so that the resulting film including the underfillmaterial and the thermally conductive sheet still functions to relievethe stresses described in the background section.

An underfill film of an embodiment of the invention is to be disposedbetween an electrical component of an electronic device and a circuitboard of the electronic device. The electrical component is affixed tothe circuit board by solder bumps. The underfill film includes amaterial, such as a resin having relatively poor thermal conductivity,and a thermally conductive sheet. The material, like the resin, is torelax thermal stresses resulting from the difference in the CTE's of thecircuit board and the solder bumps, and from the difference in the CTE'sof the electrical component and the solder bumps.

A method of an embodiment of the invention provides a first film portionof an underfill film and that has a relatively poor thermalconductivity. A thermally conductive sheet is disposed on the first filmportion, such that the underfill film includes the thermally conductivesheet. A second film portion of the underfill film is then disposed onthe thermally conductive sheet, and which also has a relatively poorthermal conductivity. Thus, the thermally conductive sheet is disposedwithin the underfill film.

The first and the second film portions may be initially formed assheets. The top and bottom surfaces of the thermally conductive sheetmay be roughened so that they adhere to the first and the second filmportions of the underfill film. The underfill film may then be appliedto a printed circuit board for an electronic device. Solder bumps can beapplied to holes within the underfill film, which may have beenpreviously formed by stamping, for instance. An electrical component ofthe electronic device is then attached to the printed circuit board. Thesolder bumps affix the component to the circuit board, and the underfillfilm is thus positioned between the component and the circuit board.

Embodiments of the invention therefore provide advantages over the priorart. Where a printed circuit board that has relatively good thermalconductivity is employed, heat is effectively dissipated to the circuitboard from an electrical component mounted to the printed circuit board.This is because the underfill film between the component and the circuitboard includes a thermally conductive sheet, providing the underfillfilm with high thermal conductivity. Alternatively or additionally, thethermally conductive sheet may be attached to a heat sink also mountedon the printed circuit board, and/or to a chassis of the electronicdevice. Therefore, heat is alternatively or also dissipated from theelectrical component, to the thermally conductive sheet, and to finallythe heat sink and/or to the chassis.

Still other advantages, aspects, and embodiments of the invention willbecome apparent by reading the detailed description that follows, and byreferring to the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

The drawings referenced herein form a part of the specification.Features shown in the drawing are meant as illustrative of only someembodiments of the invention, and not of all embodiments of theinvention, unless otherwise explicitly indicated, and implications tothe contrary are otherwise not to be made.

FIG. 1 is a diagram of an electronic device having an underfill filmincluding a thermally conductive sheet to promote heat dissipation,according to a preferred embodiment of the invention, and is suggestedfor printing on the first page of the patent.

FIGS. 2A and 2B are a top-view diagram and a cross-sectional side viewdiagram, respectively, of an underfill film having a thermallyconductive sheet, according to an embodiment of the invention.

FIG. 3 is a diagram of an electronic device in which a thermallyconductive sheet of an underfill film is connected to a number of heatsinks to promote heat dissipation, according to an embodiment of theinvention.

FIG. 4 is a diagram of an electronic device in which a thermallyconductive sheet of an underfill film is connected to a chassis topromote heat dissipation, according to an embodiment of the invention.

FIGS. 5A and 5B are flowcharts of methods, according to varyingembodiments of the invention.

DETAILED DESCRIPTION OF THE DRAWINGS

In the following detailed description of exemplary embodiments of theinvention, reference is made to the accompanying drawings that form apart hereof, and in which is shown by way of illustration specificexemplary embodiments in which the invention may be practiced. Theseembodiments are described in sufficient detail to enable those skilledin the art to practice the invention. Other embodiments may be utilized,and logical, mechanical, and other changes may be made without departingfrom the spirit or scope of the present invention. The followingdetailed description is, therefore, not to be taken in a limiting sense,and the scope of the present invention is defined only by the appendedclaims.

FIG. 1 shows an electronic device 100, according to an embodiment of theinvention. The device 100 includes a printed circuit board 102 and anelectrical component 104 mounted to the printed circuit board 102 via anumber of solder bumps or balls 106A, 106B, . . . , 106N, collectivelyreferred to as the solder bumps 106. The solder bumps 106 may be applieddirectly to the electrical component 104, through holes within anunderfill 108 and/or a thermally conductive sheet 110, as will bedescribed. In one embodiment, there are only holes within the sheet 110,such that the solder bumps can penetrate any portions of the underfill108 as needed. The solder bumps 106 may also be applied to the printedcircuit board 102. The electrical component 104 may be a semiconductorintegrated circuit (IC), or another type of electrical component. Theelectrical component 104 is a source of heat, and thus generates heatthat needs to be dissipated. The printed circuit board 102 is moregenerally considered a circuit board.

The device 100 also includes an underfill film that is made up of theunderfill 108 and the thermally conductive sheet 110. More particularly,the underfill 108 is situated between the circuit board 102 and theelectrical component 104. The thermally conductive sheet 110 is situatedwithin the underfill 108. The underfill 108 itself is at least arelatively poor thermal conductor, and thus has poor or low thermalconductivity. For instance, the underfill 108 may be a material like aresin. For instance, the underfill may be that which is available fromSumitomo Bakelite Co., Ltd., of Tokyo, Japan.

By comparison, the thermally conductive sheet 110 itself is a goodthermal conductor, and thus has good or high thermal conductivity. Forinstance, the thermally conductive sheet 110 may be a sheet of metal,like copper, aluminum, or another metal. The thermally conductive sheet110 may also be a thermally conductive graphite sheet, such as thatwhich is available from Otsuka Electric Co., Ltd., of Osaka, Japan.

The thermally conductive sheet 110 renders the underfill film a goodthermal conductor with good thermal conductivity. As a result, heatgenerated by the electrical component 104 is thermally conducted to thethermally conductive sheet 110 of the underfill film. In one embodiment,the sheet 110 extends outwards from the underfill 108, as depicted inFIG. 1, such that the sheet 110 can dissipate the heat outwards asindicated by the arrows 114A and 114B. In another embodiment, thecircuit board 102 is itself thermally conductive to at least somedegree. Therefore, the sheet 110 can further thermally conduct the heatto the circuit board 102 itself, which may dissipate some this heatgenerated by the component 104.

Furthermore, the electronic device 100 may also include a number ofsolder bumps, or balls, 112A, 112B, 112C, and 112D, collectivelyreferred to as the solder bumps 112, mounted on the underside of theprinted circuit board 102. These solder bumps 112 also dissipate heat,as indicated by the arrows 114C and 114D. That is, the heat generated bythe electrical component 104 is thermally conducted through thethermally conductive sheet 110 of the underfill film, through theprinted circuit board, and finally to the solder bumps 112, where it canbe dissipated. In all of these ways, then, varying embodiments of theinvention promote heat dissipation.

The underfill film relaxes thermal stresses that result from thecoefficient of thermal expansion (CTE) of the printed circuit board 102being significantly different than the CTE of the solder bumps 106,and/or from the CTE of the electrical component 104 being significantlydifferent than the CTE of the solder bumps 106. In one embodiment, theCTE of the underfill 108 primarily contributes to the underfill filmproviding this functionality. However, preferably the CTE of thethermally conductive sheet 110 is substantially equal to the CTE of theunderfill 108 itself. Thus, the inclusion of the thermally conductivesheet 110 into the underfill film, along with the underfill 108, doesnot affect alter the functionality of the underfill 108 to relax thermalstresses.

The electronic device 100 may be any type of electronic device, such asa computing device, an audio/video device, and so on. As can beappreciated by those of ordinary skill within the art, typically thedevice 100 will have addition components, besides the electricalcomponent 104 of FIG. 1. As can also be appreciated by those of ordinaryskill within the art, the size and shape of the various parts of theelectronic device 100 are exaggerated and not drawn to scale in FIG. 1,for illustrative clarity.

FIGS. 2A and 2B show an underfill film 200, made up of the underfill 108and the thermally conductive sheet 110, in more detail, according to anembodiment of the invention. FIG. 2A specifically shows a top view ofthe underfill film 200. By comparison FIG. 2B specifically shows across-sectional side view of the underfill film 200.

The length from left to right of the underfill film 200 in FIG. 2A maybe about 50 millimeters (mm), with the underfill 108 having a length of25 mm, such that there are about 12½ mm of the thermally conductivesheet 110 to either side of the underfill 108. The width from top tobottom of the underfill film 200 in FIG. 2A may be about 25 mm. Thethickness of the entirety of the underfill film 200 in FIG. 2B may beabout 180 micrometers (microns), with the underfill 108 having athickness of 180 microns, and the conductive sheet 110 having athickness of about 80 microns and substantially centered from top tobottom within the underfill 108.

There are a number of holes 202A, 202B, . . . , 202N, collectivelyreferred to as the holes 202, disposed within the underfill film 200,including at least through the thermally conductive sheet 110, and alsoin one embodiment through the underfill 108. The holes 202 correspond tothe solder bumps 106 of FIG. 1. That is, the holes 202 are aligned withthe solder bumps 106, and vice-versa.

For instance, once the underfill film 200 has been applied to theprinted circuit board 102 of FIG. 1, solder can be applied within theholes to create the solder bumps 106. Thereafter, the electricalcomponent 104 of FIG. 1 can be mounted to the printed circuit board 102,by being affixed to the solder bumps 106. Each of the holes may havediameters of 320 microns, where the solder bumps themselves havediameters of 200 microns. The extra 120 microns provides a degree oflatitude in application of the solder to create the solder bumps 106, toallow for misalignment, for instance.

The thermally conductive sheet 110 can be connected to other parts andcomponents of the electronic device 100 of FIG. 1 to further promoteheat dissipation. FIG. 3 shows the thermally conductive sheet 110 of theelectronic device 100 being connected to two heat sinks 302A and 302B,collectively referred to as the heat sinks 302, according to anembodiment of the invention. Like-numbered parts between FIGS. 1 and 3operate at least substantially the same in FIG. 3 as in FIG. 1, and suchdescription is not repeated here to avoid redundancy. The heat sinks 302are part of the electronic device 100.

The heat sinks 302 are mounted on the printed circuit board 102. Becausethe thermally conductive sheet 110 is connected to the heat sinks 302,further heat dissipation is promoted. Heat generated by the electricalcomponent 104 is thermally conducted to the thermally conductive sheet110 of the underfill film, and to the heat sinks 302, where the heat canbe dissipated. The heat sinks 302 can thus be located farther away fromthe electrical component 104 than is conventional, since usually heatsinks have to be in close physical contact with the electricalcomponents that they are intended to cool. Therefore, utilizing theconductive sheet 110 is advantageous, as there may be insufficient roomon the board 102 to locate the sinks 302 close to the component 104.

FIG. 4 shows the thermally conductive sheet 110 of the electronic device100 being connected to a chassis 402, according to an embodiment of theinvention. Like-numbered parts between FIGS. 1 and 4 operate at leastsubstantially the same in FIG. 4 as in FIG. 1, and such description isnot repeated here to avoid redundancy. The chassis 402 may be part of orfor the electronic device 100. For instance, the chassis 402 may be anenclosure for the device 100, and is typically fabricated from metal, oranother type of material that has high thermal conductivity.

Because the thermally conductive sheet 110 is connected to the chassis402, further heat dissipation is promoted. Heat generated by theelectrical component 104 is thermally conductive to the thermallyconductive sheet 110 of the underfill film, and to the chassis 402,where the heat can be dissipated. Therefore, utilizing the conductivesheet 110 is advantageous, because it allows the chassis 402 to be usedfor heat dissipation purposes. It is noted that in one embodiment, thethermally conductive sheet 110 may be connected to one or more heatsinks, as in FIG. 3, in addition to a chassis, as in FIG. 4.

FIG. 5A shows a method 500, according to an embodiment of the invention.First, first and second film portions of the underfill film 200 areformed as sheets (502). The first film portion is the top part of theunderfill 108 of FIG. 1, for instance, between the electrical component104 and the thermally conductive sheet 110. The second film portion isthe bottom part of the underfill 108 of FIG. 1, for instance, betweenthe thermally conductive sheet 110 and the printed circuit board 102.Next, in one embodiment, the top and bottom surfaces of the thermallyconductive sheet 110 are roughened (504). Roughening promotes subsequentadhesion of these surfaces of the conductive sheet 110 to the first andsecond film portions of the underfill film 200, especially where thesheet 110 is a copper sheet.

Thus, the first film portion of the underfill film 200 is provided(506), and the thermally conductive sheet 110 is disposed onto the firstfilm portion (508), such that it adheres thereto, and such that theunderfill film 200 is said to include the sheet 110. The second filmportion of the underfill film 200 can then be disposed onto thethermally conductive sheet 110 (510), such that it adheres thereto, andsuch that the underfill film 200 now includes the underfill 108 and theconductive sheet 110 as depicted in FIG. 1. The holes 202 may then beformed within the underfill film 200 (512), for solder to besubsequently disposed therein, and the film 200 may be applied to theprinted circuit board 102 (514). The holes 202 may be formed by astamping process, or another type of conventional or unconventionalprocess.

Thereafter, the solder bumps 106 are applied to the printed circuitboard 102 within the holes 202 within the underfill film 200 (516). Theelectrical component 104 is finally attached to the printed circuitboard 102 (518). That is, the solder bumps 106 affix the electricalcomponent 104 to the printed circuit board 102, with the underfill film200 positioned between the component 104 and the circuit board 102. Ashas been described, the presence of the thermally conductive sheet 110within this film 200 thus promotes greater heat dissipation from theelectrical component 104, in a variety of different ways.

FIG. 5B shows another embodiment of the method 500, according to adifferent embodiment than that of FIG. 5A. Like-numbered componentsbetween FIGS. 5A and 5B are performed at least substantially the same inFIG. 5B as in FIG. 5A, and the discussion thereof in relation to FIG. 5Ais not repeated in relation to FIG. 5B to avoid redundancy. The method500 as depicted in FIG. 5B is thus similar to the method 500 as depictedin FIG. 5A, with some differences.

As before, first and second film portions of the underfill film 200 areformed as sheets (502). However, next, in one embodiment the holes 202are formed within the thermally conductive sheet 110 (520), and thus arenot formed within the first and second film portions of the underfillfilm 200. This is because the solder bumps 106 that will be aligned withthe holes 202 are able to penetrate the first and second film portionsof the underfill film 200. The holes 202 may be formed by a stampingprocess, or another type of conventional or unconventional process.

Next, in one embodiment, the top and bottom surfaces of the thermallyconductive sheet 110 are roughened (504). The first film portion of theunderfill film 200 is provided (506), and the thermally conductive sheet110 is disposed onto the first film portion (508), such that it adheresthereto, as before, and such that the underfill film 200 is said toinclude the sheet 110. The second film portion of the underfill film 200can then be disposed onto the thermally conductive sheet 110 (510), suchthat it adheres thereto, and such that the underfill film 200 nowincludes the underfill 108 and the conductive sheet 110 as depicted inFIG. 1.

The film 200 may now be applied to the printed circuit board 102 (514).Thereafter, the solder bumps 106 are applied to the electrical component104, as opposed to the printed circuit board 102 as in FIG. 5A, withinthe holes 202 (22). The electrical component 104 is finally attached tothe printed circuit board 102 (518). That is, the solder bumps 106 affixthe electrical component 104 to the printed circuit board 102, with theunderfill film 200 positioned between the component 104 and the circuitboard 102. As has been noted, the presence of the thermally conductivesheet 110 within this film 200 thus promotes greater heat dissipationfrom the electrical component 104, in a variety of different ways.

It is noted that, although specific embodiments have been illustratedand described herein, it will be appreciated by those of ordinary skillin the art that any arrangement calculated to achieve the same purposemay be substituted for the specific embodiments shown. This applicationis thus intended to cover any adaptations or variations of embodimentsof the present invention. Therefore, it is manifestly intended that thisinvention be limited only by the claims and equivalents thereof.

1. A method comprising: forming as a sheet a first film portion of anunderfill film, the first film portion having a poor thermalconductivity relative to a thermally conductive sheet of the underfillfilm, the first film portion being a resin material, the thermallyconductive sheet being a thermally conductive graphite sheet; forming asa sheet a second film portion of the underfill film, the second filmportion having the poor thermal conductivity relative to the thermallyconductive sheet, the second film portion being a resin material;forming a plurality of holes within the thermally conductive sheet suchthat no holes are formed within the first film portion and within thesecond film portion, the holes formed by a stamping process; rougheninga top surface and a bottom surface of the thermally conductive sheet topromote subsequent adhesion of the top and the bottom surfaces to thefirst and the second film portions of the underfill film; disposing thethermally conductive sheet onto the first film portion such that thethermally conductive sheet adheres to the first film portion; disposingthe second film portion on the thermally conductive sheet, such that thethermally conductive sheet adheres to the second film portion, such thatthe thermally conductive sheet is sandwiched between the first and thesecond film portions, and such that the underfill film includes thefirst film portion, the thermally conductive sheet, and the second filmportion, where the thermally conductive sheet has a coefficient ofthermal expansion (CTE) substantially equal to a CTE of the underfillfilm, such that inclusion of the thermally conductive sheet within theunderfill film does not affect a functionality of the underfill film torelax thermal stresses; applying the underfill film to a printed circuitboard; applying the solder bumps to an electrical component; and,attaching the electrical component to the printed circuit board suchthat the underfill film is positioned between the electrical componentand the printed circuit board, such that the solder bumps correspond toand are aligned to the holes within the thermally conductive sheet, andsuch that the solder bumps penetrate the first film portion and thesecond film portion of the underfill film, wherein presence of thethermally conductive sheet within the underfill film promotes greaterheat dissipation from the electrical component as compared to where thethermally conductive sheet is not included within the underfill film.